The second stage motor casing consists of a cylindrical section, upper bottom and lower bottom block. The cylindrical section with wall thickness of 1.7-1.9 mm is made by extrusion method on a lathe-hydrodrater, and the top and bottom bottom bottom - by precision hot stamping. The upper bottom is welded to the body and the lower one is bolted. The casing has small skirts for attaching to adapters between steps. Outside the casing there is a fairing that covers the wiring from the pointing system to the nozzle drive control unit.
The inner walls of the casing are covered with an insulating gasket, which is a preformed erosion-resistant rubber, the thickness of the gasket gradually increases from the head end. The minimum thickness of the gasket is 2.5 mm, the maximum thickness is 25 mm. The gasket is applied to the walls of the body by vulcanizing.
Externally, the second stage engine housing is given the same protective coating as the first stage. Its thickness is different depending on the calculated thermal loads on the different parts of the casing.
At the bottom bottom of the casing there are four short exhaust pipes connecting the combustion chamber to the nozzles. The pipes inside are graphite insulated on a gasket made of glass fibre-reinforced resin. The outer and inner sides of the bottom floor are covered with rubber insulating gaskets by the vulcanizing method to protect the bottom from overheating during engine operation.
The finished engine housing without the bottom bottom bottom is installed vertically in the vacuum chamber for fuel filling. This chamber is placed in a special well, the cover of which is at floor level.
The second stage engine fuel is mixed and consists of polyurethane, ammonium perchlorate and aluminum powder. The fuel is mixed in portions of approximately 1000 kg each. A total of five such portions are required. The fuel is filled in such a way that a two-layer charge is formed, and the outer layer of fuel has a lower combustion rate. When this type of charge burns, the time pressure curve is close to optimum. Pouring in two layers of fuel increases the cost of producing engines, but this increase is fully compensated by improved engine performance.
Before pouring, a rod is placed in the housing, the shape of which corresponds to a cavity in the outer fuel layer. After pouring and hardening the outer layer, the rod is removed and another one is placed in its place, which serves to form a cavity in the inner fuel layer. The cross section of this rod has a four-blade shape. After pouring both layers of fuel it is polymerized within 4-6 days. The fuel is also poured on the bottom bottom, forming an insulating layer and simultaneously creating additional traction.
At the end of polymerization, the excess fuel is cut off from the end of the charge. A sponge gasket is placed between the two to compensate for small variations in the size of the main fuel charge and the charge cast in the bottom.
The finished engine is X-ray tested. The firm guarantees a three-year period of fuel characteristics permanence, however, it is indicated that this period is actually longer.